The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus capable of forming a high-quality image by means of causing toner to adhere onto an image forming face at uniform density through transfer.
An image forming apparatus of electrophotographic recording type which forms an electrostatic latent image on the surface of an image carrier formed from a photosensitive member by means of exposure has hitherto been known. In this image forming apparatus, the electrostatic latent image is developed with toner, and a resultant toner image is carried by the surface of an image carrier. The toner image is then transferred to a recording medium, such as a recording sheet, to thus form an image. The electrostatic latent image is subjected to toner development by means of rotating a development roller opposing the surface of the image carrier, to thus cause the toner to transfer and adhere to an outer peripheral surface of the development roller so as to develop the electrostatic latent image with the toner. A supply roller is rotated in a toner housing space in a manner that the supply roller is compressed to the development roller so that the toner is supplied to the development roller.
In an image forming apparatus adopting this electrophotographic recording type, an image is formed by transferring a toner image, which has been formed by developing an electronic latent image on the surface of a image carrier, in its present form on a recording medium, and fusing the thus-transferred toner image. The thickness of the toner laid over the outer peripheral surface of the development roller, which is used for developing the electrostatic latent image on the surface of the image carrier, greatly affects the density of the image on the recording medium. For this reason, this image forming apparatus is provided with a control member for rendering the thickness of the toner that is applied to and caused to adhere to the outer peripheral surface of the development roller uniform. This control member is disposed in parallel to the outer peripheral surface of the development roller, and controls the thickness of the toner so as to become uniform by means of scraping the toner.
Here, opposite ends of the development roller are rotatably supported. Accordingly, when the development roller is pressed by the member (e.g. the supply roller) parallel to the axial direction, a center portion thereof becomes bowed so as to escape. The center portion also becomes deflected and bowed with respect to the control member that should be disposed in parallel to the outer peripheral surface of the development roller. As a result of a recent improvement in metallic material, the metallic material is changed from, e.g., a steel material to aluminum, for making the development roller lightweight. Further, the structure of the development roller is changed from a cylindrical (solid) highly-rigid structure to a cylindrical (hollow) low-rigidity structure. In association with these changes, the tendency of deflection becomes greater.
The deflection of this development roller makes it difficult for the control member to render the thickness of the toner adhering to the outer peripheral of the development roller uniform. As a result, difficulty is encountered in rendering the density of the toner image formed by developing the electrostatic latent image on the surface of the image carrier uniform. Consequently, when that toner image is transferred and fused, in its present form, onto the recording medium, the non-uniform thickness of the toner on the outer peripheral surface of the development roller deteriorates the quality of an image formed on the recording medium.
Therefore, the following has been proposed with respect to the image forming apparatus adopting an electrophotographic recording type. Namely, the control member is formed into an outwardly-bowed shape so as to follow the curve of the outer peripheral surface of the development roller, or the center portion of the control member is pressed to bend the control member toward the development roller, thereby making the thickness of the toner on the outer peripheral surface of the development roller uniform (see, e.g., JP-A-6-110311).
In such an image forming apparatus, so-called patch processing is performed for checking whether or not a bias voltage used for causing the toner to transfer from the development roller to the surface of the image carrier has been adjusted so as to be able to form image from a fine-line image to a solid image with high quality. By means of the patch processing, a patch image is formed as a dummy image, and the density of the toner of the patch image is detected by a patch sensor, whereby the bias voltage is ascertained and adjusted (see, e.g., JP-A-2004-77873).
However, this related image forming apparatus (e.g., JP-A-6-110311) has such a structure that a flat portion of the control member, which is located closer to a rearward end than a leading-side edge, is brought into compressed contact with the outer peripheral surface of the development roller, thereby scraping the toner adhering to the outer peripheral surface by use of the leading-side edge to thus control the thickness of the toner to a fixed value. When the leading-side edge of the control member protrudes so as to depart, in a tangential direction of the developing roller, from the location where the control member is brought into compressed contact with the outer peripheral surface of the development roller, the layer of the toner that enters a space between the outer peripheral surface of the development roller and the leading-side edge becomes thicker.
Here, the development roller becomes deflected as a result of being brought into compressed contact with the supply roller. In contrast, the control member is brought into compressed contact with the outer peripheral surface of the development roller at a position deviating from the compressed contact position. It is difficult to align the control member with the curve of the outer peripheral surface of the development roller with high accuracy. For these reasons, as in the case of the image forming apparatus described in JP-A-6-110311, even when the control member is bent, the center portion of the bent control member approaches the outer peripheral surface of the development roller that is bent by being brought in contact with the supply roller, from a direction slightly different from the direction in which the development roller is bent (see FIG. 5A).
Consequently, as compared with opposite ends of the control member, the center portion of the leading-side edge of the control member protrudes so as to separate, in the tangential direction, from the outer peripheral surface of the development roller, thereby becoming displaced in a direction in which the layer of toner becomes thick. Thus, the thickness of the toner on the outer peripheral surface of the development roller cannot effectively be made uniform in the axial direction thereof.
Moreover, when the thickness of the toner on the outer peripheral surface of the development roller is non-uniform with respect to the axial direction and uncontrollable, a correlation between the density of the toner image of the patch image and the density of the toner image formed in another area is uncertain. Therefore, even when the bias voltage is adjusted by reference to the density of the toner image located at the position where the patch image is formed (the position where the patch sensor performs detection), development of the toner image performed during operation cannot be optimized.